Thermally conductive adhesive composition and process for device attachment

ABSTRACT

A thermally conductive adhesive composition includes a powder of a high melting point metal or metal alloy, a powder of a low melting point metal or metal alloy, and a polymerizable fluxing polymer matrix composition having a polyepoxide polymer resin and a low-melting solid or liquid acid-anhydride and a polymer diluent or diluents with carbon carbon double bonds and/or functional hydroxyl groups. The ratio by weight of the low melting point powder to high melting point powder ranges from about 0.50 to about 0.80, and may range from about 0.64 to about 0.75, and may be 0.665. Heretofore unpredicted substantially higher thermal conductivity improvements in performance have been found using these ratios of low melting point powder to high melting point powder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/550,408, filed Sep. 23, 2005, abandoned, which is an entryunder 35 U.S.C. 371 of International Patent Application No.PCT/US04/09886 filed Mar. 30, 2004. Like International PatentApplication No. PCT/US04/09886, the present application claims thebenefit of U.S. Provisional Patent Application No. 60/458,944, filedApr. 1, 2003. The present application also claims the benefit of U.S.Provisional Patent Application No. 60/902,057, filed Feb. 20, 2007. Thecontents of all of the above applications are hereby incorporated byreference in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present subject matter relates to the use of conductive adhesivesfor fabrication of electronic assemblies. More specifically it relatesto materials, methods and assemblies for the fabrication of electronicscontaining devices requiring thermal dissipation for cooling. Thepresent subject matter also relates to adhesives for semiconductor dieattachment that provide improved thermal dissipation.

BACKGROUND OF THE INVENTION

In order for a thermally conductive adhesive composition to be useful inthe manufacture of semiconductor devices, it should meet certainperformance, reliability and manufacturing requirements dictated by theparticular application. Such performance properties include strength ofadhesion, coefficient of thermal expansion, flexibility, temperaturestability, moisture resistance, electrical and thermal conductivity andthe like. Thermal conductivity is of particular importance in theelectronics industry. With the trend towards miniaturization coupledwith higher operating frequencies, there are ever-increasing demands onengineers to remove heat from circuitry. The extraction of heatgenerated by components within a package is necessary to prevent thosecomponents from overheating. This is a larger problem for electronicscontaining high-power devices that can dissipate many watts of energyduring normal operation.

In the prior art, die attachment adhesives generally comprised a silverflake or powder dispersed in a curable resin, such as an epoxy. However,such prior-art adhesives have thermal conductivities unsuitable fordevices that dissipate large amounts of heat. Additionally, the priorart adhesives often have poor mechanical properties. Anotherdisadvantage is that some prior art adhesives contain solvents tomaintain low viscosity. During cure, such solvents have a propensity toform voids, requiring a long bake-out operation to drive off the solventprior to cure. This adds time and cost to the overall cure process.Another shortcoming is that adhesives generally have unstable contactresistance after environmental aging. Heat and humidity also tends toreduce adhesion of conductive adhesives. Moisture absorption ofconductive adhesives can lead to delamination failures during printedcircuit assembly.

Few prior-art die attachment adhesives have the thermal conductivitysuitable for use with high power devices. As a result, solder bonding isoften the selected method. Solders have the advantage of having manytimes the thermal conductivity of most die attachment adhesives. Soldersalso have the advantage of the solder forming intimate metallurgicalbonds with the devices being soldered. A metallurgical interfaceprovides superior heat transfer compared to the typical adhesiveinterface.

However, solder bonding has a number of disadvantages. Solder preformsare usually employed to dispense solder between devices to be bonded,which are more expensive to apply during production than adhesivepastes. Another difficulty is that solder remelts if heated to anelevated temperature, yet elevated temperatures are required duringelectronic fabrication, e.g. during assembly of components to printedcircuit boards. Such remelting of solder between components in a circuitcan cause the parts to separate and subsequently fail.

Art related to adhesives is found in U.S. Pat. Nos. 6,613,123,6,528,169, 6,238,599, 6,140,402, 6,132,646, 6,114,413, 6,017,634,5,985,456, 5,985,043, 5,928,404, 5,830,389, 5,713,508, 5,488,082,5,475,048, 5,376,403, 5,285,417, 5,136,365, 5,116,433, 5,062,896, and5,043,102. Representative art directed to die attachment is found inU.S. Pat. Nos. 4,811,081, 4,906,596, 5,006,575, 5,250,600, 5,386,000,5,399,907, 5,489,637, 5,973,052, 6,147,141, 6,242,513, and 6,351,340,and published PCT application WO 98/33645. The entire contents of alllisted documents is hereby incorporated by reference.

There is clearly a need for a new composition that provides the bestadvantages of both solder and conductive adhesive. There is a need for aconductive adhesive that not only forms metallurgical bonds with thedevices being bonded, but also provides significantly more thermalconductivity than is currently possible with silver powder-resincompositions while retaining high mechanical strength. Moreover, thereis a need for a bonding material that hardens when used so that it doesnot remelt at elevated temperatures as well as possessing a high thermalconductivity, yet can be dispensed in paste form, without solvents,rather than preforms. Additionally, there is a need for a conductiveadhesive that does not suffer delamination, reduced adhesion, orconductivity after aging, humidity exposure, etc.

BRIEF SUMMARY OF THE INVENTION

The present inventive subject matter relates to a thermally conductiveadhesive composition comprising:

a powder of a high melting point metal or metal alloy;

a powder of a low melting point metal or metal alloy; and

a polymerizable fluxing polymer matrix composition comprising apolyepoxide polymer resin and a low-melting solid or liquidacid-anhydride, wherein the polymerizable fluxing polymer matrixcomposition further comprises:

(A) a polymer diluent with at least one carbon carbon double bond, andanother polymer diluent with at least one functional hydroxyl group, or

(B) a polymer diluent with both at least one carbon-carbon double bondand at least one functional hydroxyl group.

In some aspects, the ratio by weight of the powder of the low meltingpoint metal or metal alloy to the powder of the high melting point metalor metal alloy ranges from about 0.50 to about 0.80, and in some aspectsthe ratio is from about 0.64 to about 0.75, and in some aspects theratio is 0.665. The applicants have found unexpectedly heretoforeunpredicted substantially higher thermal conductivity improvements inperformance using these ratios of low melting point powder to highmelting point powder.

In some embodiments, the composition comprises 50%-60% by weight thepowder of the high melting point metal or metal alloy, and 30%-40% byweight the powder of the low melting point metal or metal alloy.

In some embodiments, the acid-anhydride comprises at least one materialselected from the group consisting of: tetrahydrophthalic anhydride,hexahydrophthalic anhydride, nadic methyl anhydride,4-methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydrideand mixtures thereof.

In some embodiments, the polymer diluent with at least one carbon carbondouble bond comprises at least one material selected from the groupconsisting of: 1,6 hexanediol diacrylatel; 1,6-hexanedioldimethacrylate; tris[2-(acryloxy)ethyl]isocyanurate; trimethylolpropanetrimethacrylate; ethoxylated bisphenol diacrylate and mixtures thereof.

In some embodiments, the polymer diluent with at least one functionalhydroxyl group comprises at least one material selected from the groupconsisting of: triethylene glycol, glycerol, tri(propylene glycol)methylether, di(ethylene glycol)butyl ether.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises a source of radical initiators. In someembodiments, the source of radical initiators comprises at least onematerial selected from the group consisting of: azobiscyclohexanecarbontrile, benzoyl peroxide, cumyl peroxide,1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azbbisisobutyronitrile, andmixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises an inerting agent. In some embodiments,the inerting agent comprises at least one material selected from thegroup consisting of bisphenol A diglycidyl ether, bisphenol F diglycidylether, 1,4-cyclohexanedimethanol diglycidyl ether,3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexaniecarboxylate,N,N-diglycidyl-4-glycidyl-oxyaniline, glycidyl phenyl ether, glycidyl4-methoxyphenyl ether, epoxy propyl benzene and mixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises an amine curing additive.

In some embodiments, the powder of the high melting point metal or metalalloy comprises electronic grade copper. In some embodiments, the powderof the high melting point metal or metal alloy comprises a materialselected from the group consisting of copper, silver, aluminum, nickel,gold, platinum, palladium, beryllium, rhodium, nickel, cobalt, iron,molybdenum and alloys and mixtures thereof.

In some embodiments, the powder of the low melting point metal or metalalloy comprises a material selected from the group consisting of Sn, Bi,Pb, Cd, Zn, In, Te, Tl, Sb, Se, Ag, and alloys and mixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises a polymerizable fluxing agent representedby the formula RCOOH, wherein R comprises a moiety having one or morepolymerizable carbon-carbon double bonds. In some embodiments, thepolymerizable fluxing agent comprises a material selected from the groupconsisting of 2-(methacryloyloxy)ethyl succinate,mono-2-(methacryloyloxy)ethyl maleate, mono-2-(methacryloyloxy)ethylphthalate, mono-2-(acryloyloxy)ethyl succinate and mixtures thereof.

In some embodiments, the polymer diluent with at least one functionalhydroxyl group is present in sufficient amount to catalyzeesterification of the acid anhydride so as to produce a polymerizablefluxing agent during heating of the conductive adhesive. In someembodiments, the polymer diluent with at least one functional hydroxylgroup is present in sufficient amount to aid in spreading or wetting ofthe polymerizable fluxing polymer matrix during heating of theconductive adhesive.

The present inventive subject matter is also drawn to an electronicassembly comprising an electronic device and either

(i) a substrate bonded to the electronic device by a sintered thermallyconductive adhesive; or

(ii) a substrate bonded to a heat spreading shim, where both thesubstrate and shim are bonded to each other and to the electronic deviceby a sintered thermally conductive adhesive composition.

The sintered thermally conductive adhesive comprises:

a powder of a high melting point metal or metal alloy;

a powder of a low melting point metal or metal alloy; and

a polymerizable fluxing polymer matrix composition comprising apolyepoxide polymer resin and a low-melting solid or liquidacid-anhydride, wherein the polymerizable fluxing polymer matrixcomposition further comprises:

(A) a polymer diluent with at least one carbon carbon double bond, andanother polymer diluent with at least one functional hydroxyl group, or

(B) a polymer diluent with both at least one carbon-carbon double bondand at least one functional hydroxyl group.

Again, in some aspects of these assemblies, the ratio by weight of thepowder of the low melting point metal or metal alloy to the powder ofthe high melting point metal or metal alloy ranges from about 0.50 toabout 0.80, and in some aspects the ratio ranges from about 0.64 toabout 0.75, and in some aspects the ratio is 0.665. The applicants havefound unexpectedly heretofore unpredicted substantially higher thermalconductivity improvements in performance using these ratios of lowmelting point powder to high melting point powder.

In some embodiments, the acid-anhydride comprises at least one materialselected from the group consisting of: tetrahydrophthalic anhydride,hexahydrophthalic anhydride, nadic methyl anhydride,4-methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydrideand mixtures thereof.

In some embodiments, the polymer diluent with at least one carbon carbondouble bond comprises at least one material selected from the groupconsisting of: 1,6 hexanediol diacrylatel; 1,6-hexanedioldimethacrylate; tris[2-(acryloxy)ethyl]isocyanurate; trimethylolpropanetrimethacrylate; ethoxylated bisphenol diacrylate and mixtures thereof.

In some embodiments, the polymer diluent with at least one functionalhydroxyl group comprises at least one material selected from the groupconsisting of: triethylene glycol, glycerol, tri(propylene glycol)methylether, di(ethylene glycol)butyl ether.

In some embodiments, composition further comprises a source of radicalinitiators. In some embodiments, the source of radical initiatorscomprises at least one material selected from the group consisting of:azobiscyclohexane carbontrile, benzoyl peroxide, cumyl peroxide,1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azbbisisobutyronitrile, andmixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises an inerting agent. In some embodiments,the inerting agent comprises at least one material selected from thegroup consisting of bisphenol A diglycidyl ether, bisphenol F diglycidylether, 1,4-cyclohexanedimethanol diglycidyl ether,3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexaniecarboxylate,N,N-diglycidyl-4-glycidyl-oxyaniline, glycidyl phenyl ether, glycidyl4-methoxyphenyl ether, epoxy propyl benzene and mixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises an amine curing additive.

In some embodiments, the powder of the high melting point metal or metalalloy comprises electronic grade copper. In some embodiments, the powderof the high melting point metal or metal alloy comprises a materialselected from the group consisting of copper, silver, aluminum, nickel,gold, platinum, palladium, beryllium, rhodium, nickel, cobalt, iron,molybdenum and alloys and mixtures thereof.

In some embodiments, the powder of the low melting point metal or metalalloy comprises a material selected from the group consisting of Sn, Bi,Pb, Cd, Zn, In, Te, Tl, Sb, Se, Ag, and alloys and mixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises a polymerizable fluxing agent representedby the formula RCOOH wherein R comprises a moiety having one or morepolymerizable carbon-carbon double bonds. In some embodiments, thepolymerizable fluxing agent comprises a material selected from the groupconsisting of 2-(methacryloyloxy)ethyl succinate,mono-2-(methacryloyloxy)ethyl maleate, mono-2-(methacryloyloxy)ethylphthalate, mono-2-(acryloyloxy)ethyl succinate and mixtures thereof.

In some embodiments, the polymer diluent with at least one functionalhydroxyl group is present in sufficient amount to catalyzeesterification of the acid anhydride so as to produce a polymerizablefluxing agent during heating of the conductive adhesive. In someembodiments, the polymer diluent with at least one functional hydroxylgroup is present in sufficient amount to aid in spreading or wetting ofthe polymerizable fluxing polymer matrix during heating of theconductive adhesive.

Furthermore, the present inventive subject matter is directed to amethod of attaching an electronic device to a substrate comprising thesteps of:

obtaining an electronic device with at least one bondable surface;

obtaining a substrate with a corresponding bondable surface; andoptionally obtaining a heat spreading shim with two bondable surfaces;

dispensing a thermally conductive adhesive on each bondable surface ofthe substrate, electronic device, and optionally, the heat spreadingshim, said thermally conductive adhesive composition comprising:

-   -   a powder of a high melting point metal or metal alloy;    -   a powder of a low melting point metal or metal alloy; and    -   a polymerizable fluxing polymer matrix composition comprising a        polyepoxide polymer resin and a low-melting solid or liquid        acid-anhydride, wherein the polymerizable fluxing polymer matrix        composition further comprises:        -   (A) a polymer diluent with at least one carbon carbon double            bond, and another polymer diluent with at least one            functional hydroxyl group, or        -   (B) a polymer diluent with both at least one carbon-carbon            double bond and at least one functional hydroxyl group;

placing the electronic device on the substrate which may be bonded to aheat spreading shim so the bondable surface of both the electronicdevice and/or the heat spreading shim is mated with the bonding surfaceof the substrate, thereby forming a combined assembly;

heating the combined assembly to an elevated temperature, therebycausing the powder of the low melting point metal or metal alloy toliquefy;

allowing the liquefied low melting point metal or metal alloy to sinterwith the high melting point metal or metal alloy; and simultaneouslypolymerizing the fluxing matrix

rendering the fluxing matrix inert; and

allowing the assembly to cool.

In some aspects of this method, the ratio by weight of the powder of thelow melting point metal or metal alloy to the powder of the high meltingpoint metal or metal alloy ranges from about 0.50 to about 0.80, and insome aspects the ratio ranges from about 0.64 to about 0.75, and in someaspects the ratio is 0.665. The applicants have found unexpectedlyheretofore unpredicted substantially higher thermal conductivityimprovements in performance using these ratios of low melting pointpowder to high melting point powder.

In some embodiments, the acid-anhydride comprises at least one materialselected from the group consisting of: tetrahydrophthalic anhydride,hexahydrophthalic anhydride, nadic methyl anhydride,4-methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydrideand mixtures thereof.

In some embodiments, the polymer diluent with at least one carbon carbondouble bond comprises at least one material selected from the groupconsisting of: 1,6 hexanediol diacrylatel; 1,6-hexanedioldimethacrylate; tris[2-(acryloxy)ethyl]isocyanurate; trimethylolpropanetrimethacrylate; ethoxylated bisphenol diacrylate and mixtures thereof.

In some embodiments, the polymer diluent with at least one functionalhydroxyl group comprises at least one material selected from the groupconsisting of: triethylene glycol, glycerol, tri(propylene glycol)methylether, di(ethylene glycol)butyl ether.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises a source of radical initiators. In someembodiments, the source of radical initiators comprises at least onematerial selected from the group consisting of: azobiscyclohexanecarbontrile, benzoyl peroxide, cumyl peroxide,1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azbbisisobutyronitrile, andmixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises an inerting agent. In some embodiments,the inerting agent comprises at least one material selected from thegroup consisting of bisphenol A diglycidyl ether, bisphenol F diglycidylether, 1,4-cyclohexanedimethanol diglycidyl ether,3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexaniecarboxylate,N,N-diglycidyl-4-glycidyl-oxyaniline, glycidyl phenyl ether, glycidyl4-methoxyphenyl ether, epoxy propyl benzene and mixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises an amine curing additive.

In some embodiments, the powder of the high melting point metal or metalalloy comprises electronic grade copper. In some embodiments, the powderof the high melting point metal or metal alloy comprises a materialselected from the group consisting of copper, silver, aluminum, nickel,gold, platinum, palladium, beryllium, rhodium, nickel, cobalt, iron,molybdenum and alloys and mixtures thereof.

In some embodiments, the powder of the low melting point metal or metalalloy comprises a material selected from the group consisting of Sn, Bi,Pb, Cd, Zn, In, Te, Tl, Sb, Se, Ag, and alloys and mixtures thereof.

In some embodiments, the polymerizable fluxing polymer matrixcomposition further comprises a polymerizable fluxing agent representedby the formula RCOOH wherein R comprises a moiety having one or morepolymerizable carbon-carbon double bonds. In some embodiments, thepolymerizable fluxing agent comprises a material selected from the groupconsisting of 2-(methacryloyloxy)ethyl succinate,mono-2-(methacryloyloxy)ethyl maleate, mono-2-(methacryloyloxy)ethylphthalate, mono-2-(acryloyloxy)ethyl succinate and mixtures thereof.

In some embodiments, the polymer diluent with at least one functionalhydroxyl group is present in sufficient amount to catalyzeesterification of the acid anhydride so as to produce a polymerizablefluxing agent during heating of the conductive adhesive. In someembodiments, the polymer diluent with at least one functional hydroxylgroup is present in sufficient amount to aid in spreading or wetting ofthe polymerizable fluxing polymer matrix during heating of theconductive adhesive.

In some embodiments, the method includes the step of catalyzingesterification of the acid anhydride by way of the polymer diluent withat least one functional hydroxyl group so as to produce a polymerizablefluxing agent during heating of the conductive adhesive.

DETAILED DESCRIPTION OF THE INVENTION

Unlike the prior art, the adhesive formulation of the present subjectmatter produces an unexpectedly and heretofore unpredicted substantiallyhigher thermal conductivity within a narrow range of metallic loadinglevels. Ratios by weight of the powder of the low melting point metal ormetal alloy to the powder of the high melting point metal or metal alloyranges from about 0.64 to about 0.75, and significant and substantiallyhigher thermal conductivity has been observed as a ratio of about 0.665.The applicants have found unexpectedly heretofore unpredictedsubstantially higher thermal conductivity improvements in performanceusing these ratios of low melting point powder to high melting pointpowder. In this sense, the adhesives bond similarly to prior-art soldersused in die-attachment. The inventive adhesives comprise low meltingpoint fillers that, when heated, first melt, then alloy with highmelting point fillers, then harden. Thereafter the adhesives do notremelt if they are elevated to the temperature at which they firstmelted. The subject matter addresses many of the shortcomings of priorart solders and adhesives, providing an easily-processed, solvent-freeadhesive capable of forming metallurgical joints similar to solder withunexpected heretofore unpredicted substantially higher thermalconductivity. The inventive compositions have the further advantage theymay be used as a replacement for solder paste during surface mount (SMT)manufacturing. The subject matter further comprises an electronicassembly employing inventive adhesive compositions for improved thermaldissipation.

The compositions of the present subject matter are free of fugitivesolvents and comprise

a powder of a high melting point metal or metal alloy;

a powder of a low melting point metal or metal alloy; and

a polymerizable fluxing polymer matrix composition comprising apolyepoxide polymer resin and a low-melting solid or liquidacid-anhydride, wherein the polymerizable fluxing polymer matrixcomposition further comprises:

(A) a polymer diluent with at least one carbon carbon double bond, andanother polymer diluent with at least one functional hydroxyl group, or

(B) a polymer diluent with both at least one carbon-carbon double bondand at least one functional hydroxyl group.

Sintering and curing of the inventive compositions are achieved byheating. When the compositions are heated to the liquidus or meltingpoint of the low melting point component, the composition forms atransient liquid phase. Unlike the art taught by U.S. Pat. No.6,613,123, the included thermally curable adhesive flux compositionserves initially as a fluxing agent, facilitating the removal of oxidesfrom the surfaces of the metal powders, and also facilitating wetting ofmetallic surfaces by the molten metals. As the heating process iscontinued, the liquid phase and the high melting point metals react andisothermally solidify through a process known in the art as liquid-phasesintering. The heating process also serves to neutralize the fluxingcomponents in the resin so that the components become non-corrosive andchemically stable. Unlike compositions such as described in U.S. Pat.No. 5,376,403, this may occur before, during or after the sintering ofthe metals. After the sintering process occurs, the heat causes thethermally curable adhesive flux composition to polymerize, forming ahard intractable binder. Heating is done by either continuous reflowprocesses commonly used in soldering or by using simple isothermalprocessing methods.

Preferably, the primary fluxing agent in these compositions integrateswithin a single molecule carboxylic acid group that provides the fluxingaction for the soldering process without need of corrosive ions orhalogens, and can polymerize upon application of heat, to form ahigh-strength solid adhesive polymer. This is accomplished withoutgenerating gases, water, or other harmful by-products. An inerting orneutralizing agent may be included to react during heating with the fluxacid groups and any flux residues. As a consequence, after the thermallycurable adhesive composition is cured, the flux residues do not need tobe washed away or removed since they are inert and non-corrosive.

Solvents are not required as the thermally curable adhesive fluxcomposition itself can comprise a relatively low-viscosity liquid. Byincorporating low-viscosity fluxing agents, resins and diluents, thethermally curable adhesive flux composition has sufficiently lowviscosity to permit the incorporation of very high levels of conductivefiller powders without the need to add solvents.

Adhesive compositions involving transient liquid phase sintering in thepresence of a polymerizing flux are known in the prior art, for exampleU.S. Pat. No. 5,376,403. However, the prior art has been principallydirected at electrically conductive adhesives with high electricalconductivity, e.g. electrically conductive traces for printed circuits,where creation of microvoids during curing is generally harmless. Theuse of such adhesives in high thermal conductivity applications, such assilicon die attachment, had been previously stymied by the microvoidscreated in the adhesives of the prior art during the curing process.Voids cause the bonds formed to weaken. Voids also reduce the thermalconductivity of the bonds.

It has been discovered that the voids are due to fugitive solvents inthe adhesives of the prior art, e.g. butyl carbitol (see examples 1-16of U.S. Pat. No. 5,376,403), which cannot completely bake out duringcuring. These fugitive solvents have been required in the prior art inorder to make the prior art compositions completely sinter. However, inthe instant subject matter, it is possible to produce transient liquidphase sintered adhesives without fugitive solvents. The elimination ofthe fugitive solvents produces bonds that are void-free. Thus, apractical method for bonding two parts by means of transient liquidphase sintered adhesives to establish improved thermal conductivitythrough the bond is achieved. In the instant subject matter, theinventors unexpectedly discovered unprecedented and significantincreases in thermal conductivity within a narrow range of metallicloading levels.

1. Fluxing Agents

Fluxing agents normally comprise carboxylic acid moieties or precursorsof such moieties. In some presently disclosed aspects, the fluxcomprises carboxylic acid moieties. The adhesion, mechanical integrity,and corrosion resistance achieved with these fluxing agents are superiorto those achieved with some prior art polymer fluxing agents becausethere is no need to add aggressive fluxing activators. The fluxingagents can be fully crosslinked and all components chemicallyimmobilized upon curing. Even the reaction by-products of fluxdeoxidization of the metals are chemically bound in the polymer matrix.

Carboxylic acids function well as fluxing agents to remove oxides frommetals. In addition, carboxylic acids are also very effectivecrosslinking moieties when present in their reactive form in a fluxingcomposition containing a suitable thermosetting resin, such as an epoxy.For this reason, in the prior art, chemical protection of the carboxylicacid was essential to achieving stability and preventing prematurereactions, as described in U.S. Pat. No. 5,376,403. Protection wasachieved by binding the fluxing agent with a chemically- orthermally-triggered species so that it becomes reactive only at or nearthe time that the solder melts. However, with the fluxing agents of theinstant subject matter, no such protection is necessary because thecompositions do not cure significantly until the elevated temperaturerequired for sintering is reached or exceeded. This results in a fluxingagent that can function at its full strength with the metal oxides toproduce fluxing that is superior to any heretofore polymerizable fluxingagent. For die attachment adhesive applications, this allows theadhesive composition to produce sound and complete metallurgical bondswith the metallizations on the die and substrate before hardening. Thisleads to superior thermal conductivity through the bonds, not possiblein the prior art.

In some embodiments, including examples 1, 3, 5, and 6 given below, nocarboxyl fluxing agents are initially provided in preparation of thecomposition. Rather, carboxyl fluxing agents come into being atintermediary steps during heating and application of the composition,such as during esterification of a hexahydrophthalic anhydride. Theesterification is catalyzed by hydroxyls present in the mixture, such asthose provided by triethylene glycol, glycerol, tri(propyleneglycol)methyl ether, or di(ethylene glycol)butyl ether, as non-limitingexamples. Esterification is also catalized by carboxyl groups formedduring the heating and application of the composition. In general, anycarboxyl fluxing agent initially added to the composition can serve as acatalyst for these reaction. In addition, tri(propylene glycol)methylether, or di(ethylene glycol)butyl ether can also serve as fluxcarriers, aiding in flux spreading/wetting during a soldering process.

2. Inerting Agents

An inerting or neutralizing agent is added to the inventive compositionsto react with carboxylic acid present in the mixture after the fluxingaction is completed, thereby eliminating the need for additionalcleaning to remove potentially corrosive residues. Epoxides areparticularly suitable for this purpose, though others, such as cyanateesters, can also neutralize the carboxylic acid function. The reactionbetween epoxides and carboxylic acids is well known to those skilled inthe art. To ensure complete neutralization, a stoichiometric equivalentor excess of non-fluxing epoxide may be present. The inerting agent isoptionally miscible with the fluxing agent and with other components inthe composition. It can be mono-functional or multi functional, liquidor solid. Non-limiting examples of inerting agents include one or morecomponents selected from the group comprising bisphenol A diglycidylether, bisphenol F diglycidyl ether, 1,4-cyclohexanedimethanoldiglycidyl ether, 3,4-epoxycyclohexylmethyl3,4-epoxycyclohexanecarboxylate, N,N-diglycidyl-4-glycidyl-oxyaniline,glycidyl phenyl ether, glycidyl 4-methoxyphenyl ether, epoxy propylbenzene and mixtures thereof. These are all commercially available.

The inerting agent concentration in the inventive flux may bestoichiometric, or somewhat in excess of stoichiometric, with thecarboxylic acid component, in order to inert all of the acid duringcuring of the inventive conductive adhesives. Too high a concentrationof inerting agent may cause excessive polymerization, which will limitsintering of the metals, whereas too low a concentration may leaveunreacted pendant acid groups after cure, which are corrosive.

3. Resins

The thermally curable fluxing composition does not typically requireadditional non-fluxing or non-diluent resins. Compositions that do notinclude resins often have longer pot lives and lower viscosities duringsolder reflow. As a result, inclusion of a resin in the composition isnot needed, except as an inerting agent. Resins, however, can alsofunction to increase the adhesion of the cured composition to thesubstrate and to increase the cohesive strength and glass transitiontemperature of the cured composition. Thus, as an option, a resin can beemployed so long as concentrations are kept relatively low. The resinmay be any suitable resin that is blendable with the fluxing agent. Byblendable is meant that the resins do not have to be chemically bondedto the fluxing agent and/or diluent. Preferred resins, though, can reactwith the carboxylic acid groups in the fluxing agent, inerting them, orby other reactive moieties, such as optional —OH groups, in the diluent.

Non-limiting examples of resins that meet these requirements includematerials selected from the group comprising epoxies, phenolics,novalacs (both phenolic and cresolic), polyurethanes, polyimides,bismaleimides, maleimides, cyanate esters, polyvinyl alcohols,polyesters, and polyureas. Preferred resins include materials selectedfrom the group comprising bisphenol A diglycidyl ether, bisphenol Fdiglycidyl ether, 1,4-cyclohexanedimethanol diglycidyl ether,3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate,N,N-diglycidyl-4-glycidyl-oxyaniline, and mixtures thereof. These arecommercially available.

It is also beneficial to include crosslinking agents when resins areused in the inventive compositions. Crosslinking agents are wellestablished in the prior art. Examples of crosslinking agents includeanhydrides and carboxyl-functionalized polyesters. The addition of suchmaterials facilitates the crosslinking reaction of the resin. Examplesof suitable anhydride crosslinking agents include one or more componentsselected from the group of, but not limited to tetrahydroplithalicanhydride, hexahydro phthalic anhydride, nadic methyl anhydride,4-methythexahydrophthalic anhydride, and methyltetrahydrophthalicanhydride. All are commercially available.

When crosslinking agents are used, it is also useful to add anaccelerator to increase the rate of crosslinking during thermal cure.Examples of suitable accelerators include imidazole and its derivatives,dicyandiamide and biguanide derivatives as well as tertiary amines suchas benzyldimethylamine or 1,8-diazabicyclo[5.4.0]undec-7-ene.Alternatively, transition metal acetylacetonates may also be used toaccelerate the rate of reaction during thermal cure between epoxideresins and anhydride crosslinking agents. Non-limiting examples includeone or more components selected from the group comprising copper (II)acetylacetonate, cobalt (III) acetylacetonate and manganese (II)acetylacetonate.

4. Diluents

The presence of carbon-carbon double bond(s) in the fluxing polymermatrix allows significant flexibility in the formulation of a fluxcomposition with improved thermomechanical properties. This is achievedby the addition of double bond containing diluents that also crosslinkwith the flux to create a superior adhesive. This technique permits thedesign of fluxing adhesive compositions that attain high crosslinkdensities, which are desirable for good thermomechanical properties andgood adhesion. Moreover, this is accomplished without the concern ofpremature crosslinking and reduced pot life associated with the priorart. Non-limiting examples of diluents include one or more componentsselected from the group comprising 1,6-hexanediol diacrylate,1,6-hexanediol dimethacrylate, tris[2-(acryloxy)ethyl]isocyanurate,trimethylolpropane trimethacrylate, ethoxylated bisphenol diacrylate andmixtures thereof. Most di- and tri-functionalized acrylate resins withlow viscosity, well known to those skilled in the art, are suitable forthis purpose Other double bond containing compounds, many of which arecommercially available, including, for example, diallyl phthalate anddivinyl benzene can also be used. Hydrophobic diluents as described maybe used but hydrophilic diluents can also be employed when appropriate.

A benefit of employing hydrophobic diluents is that their presence tendsto reduce the amount of water that the cured adhesive composition willabsorb. The reason is that the fluxing agent, when crosslinked, willhave active carboxylic groups that can attract water, even though thesecarboxylic groups, being part of a network, are immobile. Water acts asa plasticizer, which softens the cured adhesive composition. The use ofhydrophobic diluents that are crosslinked to the fluxing agent willcounteract the hydrophilic effects of the carboxylic acid groups.

In some embodiments, two or more polymer diluents are used, where someof the polymer diluents have at least one carbon carbon double bond,while others of the polymer diluents have at least one functionalhydroxyl group. In other embodiments, at least one polymer diluent isused which has both at least one carbon-carbon double bond and at leastone functional hydroxyl group.

5. Radical Initiators

While the thermally curable adhesive composition can be cured using heatalone, the cross linking reaction can be initiated and facilitated bythe presence of free-radicals, including, for example, those generatedby initiators such as benzoyl peroxide, cumyl peroxide,1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azobisisobutyronitrile, andmixtures thereof. These free radical initiators or sources arecommercially available. In the presence of certain metals, such ascopper, premature decomposition of peroxy initiators may occur due tounfavorable redox reactions resulting in outgassing and voids in thecured composition. Therefore, in one aspect, azo-type initiators areused.

Free-radicals are created in-situ by exposure of the free-radicalinitiator to heat, radiation, or other conventional energizing sources.Introduction of an appropriate free-radical initiator accelerates theonset of crosslinking to the desired moment in a solder reflow orisothermal curing operation. The presence of a small amount offree-radical crosslinking initiator in the fluxing agent is used tocontrol the rate and the temperature of crosslinking of the fluxingagent, ensuring effective fluxing action and strong adhesion of thecomposition to the substrates upon curing.

In preparing the thermally curable adhesive flux composition, theproportions of each of the components may be varied over a considerablerange and still yield acceptable fluxing activity as well as good postcured material properties. Optionally, the thermally curable adhesiveflux composition employed does not produce gaseous byproducts thatresult in the formation of bubbles in the final cured composition.

In an aspect, the thermally curable polymeric fluxing composition, afterbeing cured, has a glass transition temperature in excess of 100° C., arelatively low coefficient of thermal expansion (100 ppm/° C. or less)and moisture uptake of less than 3%. While, again, some of the fluxingagents within these ranges exhibit high coefficient of thermal expansionor low glass transition temperature when cured, the compositions remainuseful as fluxing resins in applications where these characteristics arenot critical.

Metal Powders

The inventive adhesive compositions comprise a blend of high meltingpoint and low melting point metal or alloy powders. The powders cancomprise round particles or flakes. The metal powders should comprise arange of sizes to improve packing density. In some adhesivecompositions, the round particles have a maximum size of about 100microns and could be less than about 50 microns in size. Flakes may alsorange from about 1 to about 50 microns in size. The use of flakes belowabout 30 microns is advantageous to prevent the texture of the adhesivecomposition from becoming too coarse. These are non-limiting examples,and other sizes may be used. Though it is well known that oxide removalfrom fine metal powders is more difficult due to the higher surfacearea, the fluxing activity of the inventive compositions can besufficiently high to provide satisfactory oxide removal.

Any solderable or alloyable metal, alloy or metal mixture is usable asthe high melting point powder. For example, the high melting point metalpowder may be a material selected from the group comprising copper,silver, aluminum, nickel, gold, platinum, palladium, beryllium, rhodium,nickel, cobalt, iron, molybdenum and alloys or mixtures thereof. Inparticular, the high melting point metals may be copper, silver, nickeland/or gold. When spherical powders are used, the powders can havesmooth, even morphology, as is typically produced using gas atomizationmethods. The high melting point powder can be comprised of a mixture ofspherical powder and flake. The use of spherical powders permits a highmetal loading in the adhesive composition, which is desirable for highthermal and electrical conductivity, while the addition of flake helpsimprove the rheology of the adhesive and facilitates application ordispensing using conventional equipment used in the fabrication ofelectronic assemblies. It also serves to prevent settling of the fillerparticles in the resin, maintaining the homogeneous nature of thematerial eliminating the need to re-mix the material prior to use. Thehigh melting point powder makes up about 10-90% by weight of the totalpowder composition in an embodiment. In other aspects, the high meltingpoint powder makes up about 50-60% by weight of the total powdercomposition.

Any solderable or alloyable metal, alloy or metal mixture is usable asthe low melting point metal so long as it has a melting point well belowthat of the high melting point powder. The melting point of the lowmelting point powder is generally about 50° C. or more below the meltingpoint of the high melting point powder. Alternatively, the melting pointis about 100° C. or more below the melting point of the high meltingpoint powder. These are merely examples, and other temperaturedifferences may be selected. The low melting point metal powder cancomprise one or more elements selected from the group comprising Sn, Bi,Pb, Cd, Zn, In, Te, Tl, Sb, Se, Ag, and alloys, or mixtures thereof.These mixtures can include, but are in no way limited to, SnInAg andSnBiAg. In a particular embodiment, the low melting point powder cancomprise a commercial solder powder prepared from a combination of themetals Sn and Pb. The low melting point powder may have a liquidustemperature below 200° C. such that it melts prior to the hardening orcuring of the polymeric fluxing agent. Typically, the solder powdersused have particles sizes from about 1 to about 100 microns. This ismerely an example, however, and other sizes may be used. Commonly, thesolder powders comprise a combination of type 3, type 4 and type 5(15-45 microns) size distribution. The low melting point powder makes upabout 10-90% by weight of the adhesive powder mixture. In some aspects,the low melting point powder makes up about 30-40% by weight of thetotal powder composition.

In a particular aspect of the present subject matter, the metal powderused as the high melting point metal is electronic grade copper metal.In this aspect, the high melting point electronic grade copper may besupplied by Ultrafine Powders, Inc. of Woonsocket R.I., lot 04-277 andother lots.

In a particular aspect of the present subject matter, the ratio byweight of the powder of the low melting point metal or metal alloy tothe powder of the high melting point metal or metal alloy ranges fromabout 0.50 to about 0.80, and in some aspects the ratio ranges fromabout 0.64 to about 0.75, and in some aspects the ratio is 0.665. Theapplicants have found unexpectedly heretofore unpredicted substantiallyhigher thermal conductivity improvements in performance using theseratios of low melting point powder to high melting point powder.

Preparation of the Adhesive Compositions

In the preparation of the conductive adhesive composition, the low andhigh melting point metal powders are first blended to ensure ahomogeneous mixture. With the metal powders, the blending may beperformed in air at room temperature. Blending of the powders in aninert gas, such as nitrogen, is also possible to reduce the oxidation.Suitable methods of powder blending, such as shell blending, may also beused.

This powder mixture is then combined with the thermally curable adhesiveflux composition. High shear mixing is useful to ensure homogeneity inthe resulting paste. A method of high-shear blending known in the art isdual planetary mixing. The concentration of metal powder in the finaladhesive can range from about 80-93% by weight, or 85-92% by weight, ofthe total adhesive composition, but these are a non-limiting examplesand more or less metal powder may be used relative to the fullcomposition. The remainder of the adhesive composition, which can rangefrom about 7-20% by weight, or about 8-15% by weight, is comprised ofthe thermally curable adhesive flux composition, although again theseare nonlimiting examples. These adhesive compositions are generallypaste-like and are typically suitable for dispensing through a syringeusing commercial dispensing equipment without the need for addedsolvent. Alternatively, the adhesive compositions may be suitable forapplication by stencil or screen printing techniques, well known tothose skilled in the art.

Die Attachment

Though the thermally-curable adhesive compositions of the presentsubject matter have many uses, the adhesives are particularly wellsuited for attaching semiconductor die to substrates and optionally,heat spreading shims. In particular, the high thermal conductivity ofthe adhesives makes them well suited for bonding of semiconductor powerdevices to substrates and heat spreading shims. It is optional that thesubstrate, heat spreading shim and the die be metallized to allow thesolder or low-melting point alloy to form metallurgical bonds. Suchmetallurgical bonds provide high strength and superior thermal andelectrical conductivity.

In the prior art, semiconductor power devices are commonly bonded usingsolder. However, since the alloys employed in the inventive compositionshave lower melting points than prior-art die attachment solders, anadvantage of the inventive compositions is that metallurgical bondformation can occur at lower temperatures. Furthermore, the transientliquid phase sintering that occurs during heating results in highmelting point alloys that melt at temperatures well above the originalcuring temperature. This advantage over prior art solders providesadditional latitude in the temperatures used to perform subsequentelectronic assembly. The heat applied during the transient liquid phasesintering operation also cures the polymer flux, forming a secondaryhigh strength bond.

The thermally curable adhesive compositions of the present subjectmatter are also suitable for attaching semiconductor die to a substrateor optionally, heat spreading shims in situations where the die, thesubstrate or both have no metallization. In these instances, solder dieattachment is not possible. Adhesion of the die to the substrate or shimis then due solely to the bonds formed by the polymeric component of theinventive adhesive, as is the case with prior art die attachmentadhesives comprising a silver flake or powder dispersed in a curableresin. In these instances of the inventive die attachment processes,sintering occurs in the bulk of the disclosed adhesive, but nometallurgical bond formation occurs at the interfaces of the surfacesbeing joined. The efficiency of heat transfer through these interfacesis now reduced compared to metallized surfaces.

However, in these instances, the sintering that occurs in the bulk ofthe adhesive provides higher stability and thermal conductivity thantypically found prior art die attachment adhesives. Prior art adhesivesrely on point-to-point contact of the filler particles to providethermal and electrical conductivity. With age, this point-to-pointcontact undergoes degradation, resulting in reduced thermal andelectrical properties. Such degradation does not occur in the inventivecompositions since the filler particles are effectively sinteredtogether.

Method of Bonding

The present inventive subject matter is further directed toward a methodof attaching an electronic device to a substrate which comprises thesteps of: obtaining an electronic device, such as a silicon die, with atleast one bondable surface; obtaining a substrate with a correspondingbondable surface; and optionally, obtaining a heat spreading shim withtwo bondable surfaces; dispensing the inventive thermally conductiveadhesive on one or both of the bondable surface(s) of the substrate,shim, or electronic device; placing the electronic device on thesubstrate, or shim so the two bondable surfaces are mated, therebyforming a combined assembly; heating the combined assembly to anelevated temperature, causing the powder of the relatively low meltingpoint metal or metal alloy to liquefy; allowing the liquefied lowmelting point metal or metal alloy to sinter with the relatively highmelting point metal or metal alloy; polymerizing the fluxing matrix,thereby rendering the fluxing matrix inert; and allowing the assembly tocool.

A small amount of the presently disclosed composition is applied to thedesired bonding area on the substrate, shim, or die using conventionalsyringe dispensing equipment, known to those skilled in the art. Theadhesive is dispensed as a small dot or in any pattern. Alternately, theadhesive is stencil printed onto the parts using stencil printingequipment known in the art. Sufficient material is dispensed to ensurethe formation of a small fillet of material around the edge of the dieafter placement. Using conventional die placement equipment, the die isthen placed on the bonding area and pressed with sufficient force toensure complete coverage of the underside of the die with the adhesive.The assembly is then heated in an oven. An isothermal oven may be used,but alternatively, a multizone solder reflow oven may be employed. Ineither case, for sintering to occur, the assembly should reach the meltor liquidus temperature of the low melting point alloy before thethermally curable adhesive flux composition hardens. In some inventiveadhesive compositions, multiple passes through a reflow oven may beneeded to complete the sintering process.

The following examples are illustrative of embodiments of the subjectmatter and are not to be construed as limiting the subject matterthereto. All percentages are given in weight percent, unless otherwisenoted and equal a total of 100%.

Example 1 Die Attachment Composition

Components Amt wt % Copper Powder 12.00000 54.496% Tin-Lead EutecticPowder 7.98000 36.240% Bisphenol A Digylcidyl Ether 0.96088 4.364%Hexahydrophthalic Anhydride 0.76219 3.461% Triethylene Glycol 0.081430.370% 1,6 Hexanediol Diacrylate 0.23452 1.065%Azobiscyclohexanecarbontrile 0.00098 0.004%

Hexahydrophthalic anhydride was dissolved in Bisphenol A diglycidylether by warming the mixture to 65° C. After stirring to form ahomogeneous mixture, the blend was cooled to room temperature.1,6-hexanediol diacrylate, triethylene glycol and azobiscyclohexanecarbontrile were then added with stirring to complete thepolymer flux component of the adhesive composition. In a separatecontainer, copper powder and 63Sn37Pb solder powder were mixed, using ahand blender. The copper powder was supplied by Ultrafine Powders, Inc.of Woonsocket, R.I., lot number 04-277. This mixture of metal powderswas then added to the polymer flux. Homogeneity was achieved by highshear mixing in a mechanical blender. Finally, the mixture was degassedunder high vacuum.

The composition was dispensed into a Teflon mold and passed through a 5minute solder reflow cycle having a peak temperature of 210° C.,followed by a post cure at 200° C. for 20 minutes. Bulk thermalconductivity of the sample had a conductivity of: 43.24 W/m K.

Example 2 Die Attachment Composition

Components Amt wt % Copper Powder 12.00000 54.443% Tin-Lead EutecticPowder 7.98000 36.205% Bisphenol A Digylcidyl Ether 0.92333 4.189%Hexahydrophthalic Anhydride 0.81253 3.686% Triethylene Glycol 0.077870.353% 1,6 Hexanediol Diacrylate 0.22535 1.022% mono-2-(methacryloyloxy)ethyl maleate 0.02124 0.096% Azobiscyclohexanecarbontrile 0.00092 0.004%

Hexahydrophthalic anhydride was dissolved in Bisphenol A diglycidylether by warming the mixture to 65° C. After stirring to form ahomogeneous mixture, the blend was cooled to room temperature.1,6-hexanediol diacrylate, triethyline glycol,mono-2-(methacryloyloxy)ethyl maleate, and azo biscyclohexanecarbontrilewere then added with stirring to complete the polymer flux component ofthe adhesive composition. In a separate container, copper powder and63Sn37Pb solder powder were mixed, using a hand blender. The copperpowder was supplied by Ultrafine Powders, Inc. of Woonsocket, R.I., lotnumber 04-277. This mixture of metal powders was then added to thepolymer flux. Homogeneity was achieved by high shear mixing in amechanical blender. Finally, the mixture was degassed under high vacuum.

The composition was dispensed into a Teflon mold and passed through a 5minute solder reflow cycle having a peak temperature of 210° C.,followed by a post cure at 200° C. for 20 minutes. Bulk thermalconductivity of the sample had a conductivity of: 40.954 W/m K.

Example 3 Comparative Die Attachment Composition

Components Amt wt % Copper Powder 12.0000 54.496% Tin-Lead EutecticPowder 7.98000 36.240% Bisphenol A Digylcidyl Ether 0.98411 4.469%Hexahydrophthalic Anhydride 0.78061 3.545% Glycerol 0.03409 0.155% 1,6Hexanediol Diacrylate 0.24019 1.091% Azobiscyclohexanecarbontrile0.00100 0.005%

Hexahydrophthalic anhydride was dissolved in Bisphenol A diglycidylether by warming the mixture to 65° C. After stirring to form ahomogeneous mixture, the blend was cooled to room temperature. Glycerol,1,6-hexanediol diacrylate and azo biscyclohexanecarbontrile were thenadded with stirring to complete the polymer flux component of theadhesive composition. In a separate container, copper powder and63Sn37Pb solder powder were mixed, using a hand blender. The copperpowder was from lot number 04-277 from Ultrafine Powders, Inc. ofWoonsocket, R.I. This mixture of metal powders was then added to thepolymer flux. Homogeneity was achieved by high shear mixing in amechanical blender. Finally, the mixture was degassed under high vacuum.

The composition was dispensed into a Teflon mold and passed through a 5minute solder reflow cycle having a peak temperature of 210° C.,followed by a post cure at 200° C. for 20 minutes. Bulk thermalconductivity of the sample had a conductivity of: 34.111 W/m K.

Example 4 Comparative Die Attachment Composition

Components Amt wt % Copper Powder 12.00000 54.496% Tin-Lead EutecticPowder 7.98000 36.240% Bisphenol A Digylcidyl Ether 0.93768 4.258%Hexahydrophthalic Anhydride 0.74378 3.378% mono-2-(methacryloyloxy)ethyl maleate 0.15893 0.722% poly(ethylene glycol) methacrylate 0.198660.902% Azobiscyclohexanecarbontrile 0.00095 0.004%

Hexahydrophthalic anhydride was dissolved in Bisphenol A diglycidylether by warming the mixture to 65° C. After stirring to form ahomogeneous mixture, the blend was cooled to room temperature.mono-2-(methacryloyloxy)ethyl maleate, poly(ethylene glycol)methacrylateand azo biscyclohexanecarbontrile were then added with stirring tocomplete the polymer flux component of the adhesive composition. In aseparate container, copper powder and 63Sn37Pb solder powder were mixed,using a hand blender. The copper powder was from lot number 04-277 fromUltrafine Powders, Inc. of Woonsocket, R.I. This mixture of metalpowders was then added to the polymer flux. Homogeneity was achieved byhigh shear mixing in a mechanical blender. Finally, the mixture wasdegassed under high vacuum.

The composition was dispensed into a Teflon mold and passed through a 5minute solder reflow cycle having a peak temperature of 210° C.,followed by a post cure at 200° C. for 20 minutes. Bulk thermalconductivity of the sample had a conductivity of: 41.798 W/m K.

Example 5 Comparative Die Attachment Composition

Components Amt wt % Copper Powder 12.00000 54.496% Tin-Lead EutecticPowder 7.98000 36.240% Bisphenol A Digylcidyl Ether 0.89824 4.079%Hexahydrophthalic Anhydride 0.71250 3.236% tri(propylene glycol) methylether 0.20911 0.950% 1,6 Hexanediol Diacrylate 0.21923 0.996%Azobiscyclohexanecarbontrile 0.00091 0.004%

Hexahydrophthalic anhydride was dissolved in Bisphenol A diglycidylether by warming the mixture to 65° C. After stirring to form ahomogeneous mixture, the blend was cooled to room temperature.Tri(propylene glycol)methyl ether, 1,6 hexanediol diacrylate and azobiscyclohexanecarbontrile were then added with stirring to complete thepolymer flux component of the adhesive composition. In a separatecontainer, copper powder and 63Sn37Pb solder powder were mixed, using ahand blender. The copper powder was from lot number 04-277 fromUltrafine Powders, Inc. of Woonsocket, R.I. This mixture of metalpowders was then added to the polymer flux. Homogeneity was achieved byhigh shear mixing in a mechanical blender. Finally, the mixture wasdegassed under high vacuum.

The composition was dispensed into a Teflon mold and passed through a 5minute solder reflow cycle having a peak temperature of 210° C.,followed by a post cure at 200° C. for 20 minutes. Bulk thermalconductivity of the sample had a conductivity of: 42.498 W/m K.

Example 6 Comparative Die Attachment Composition

Components Amt wt % Copper Powder 12.00000 54.496% Tin-Lead EutecticPowder 7.98000 36.240% Bisphenol A Digylcidyl Ether 0.91836 4.171%Hexahydrophthalic Anhydride 0.72846 3.308% di(ethylene glycol) butylether 0.16811 0.763% 1,6 Hexanediol Diacrylate 0.22414 1.018%Azobiscyclohexanecarbontrile 0.00093 0.004%

Hexahydrophthalic anhydride was dissolved in Bisphenol A diglycidylether by warming the mixture to 65° C. After stirring to form ahomogeneous mixture, the blend was cooled to room temperature.Di(ethylene glycol)butyl ether, 1,6 hexanediol diacrylate and azobiscyclohexanecarbontrile were then added with stirring to complete thepolymer flux component of the adhesive composition. In a separatecontainer, copper powder and 63Sn37Pb solder powder were mixed, using ahand blender. The copper powder was from lot number 04-277 fromUltrafine Powders, Inc. of Woonsocket, R.I. This mixture of metalpowders was then added to the polymer flux. Homogeneity was achieved byhigh shear mixing in a mechanical blender. Finally, the mixture wasdegassed under high vacuum.

The composition was dispensed into a Teflon mold and passed through a 5minute solder reflow cycle having a peak temperature of 210° C.,followed by a post cure at 200° C. for 20 minutes. Bulk thermalconductivity of the sample had a conductivity of: 51.194 W/m K.

The composition labeled “Example 4” above is known by the applicants tohave exceptional pot-life characteristics above those of the otherdisclosed compositions above. The composition of Example 4 was testedfor viscosity on a Brookfield cone and plate viscometer and had aviscosity of about 100000 cps @1 rpm and 25 C. The composition wastested every two hours, and no discernable change in viscosity was foundafter 10 hours of testing. This differs from the other compositions,whose viscosity was found to double after approximately 2.5 hours atroom-temperature.

The presently disclosed subject matter includes both lead-containing andlead-free bonding materials. Although a number of examples presented inthis disclosure comprise lead, many conventional die attachment solderscontaining lead are now avoided due to environmental concerns. Typicallead-free solders, proposed as an alternative to conventional dieattachment solders containing lead, have their own problems: they oftenrequire very high process temperatures that are often damaging to theassemblies themselves, and the solder tends to remelt if heated to anelevated temperature, such as the temperatures required duringelectronic fabrication, which can cause the bonded parts to separate andsubsequently fail. Thus, the present disclosed subject matter proposesconductive adhesive materials which need not utilize lead but which inany case form metallurgical bonds with significant thermal conductivityand high mechanical strength, and which harden when used so as not toremelt at elevated temperatures. Lead-free bonding materials may requiredifferent ratios of low to high melting point metals than thosedisclosed above, including (but not limited to) ratios of about 0.9 toabout 1; about 1 to about 1; and about 1.1 to about 1, although otherratios may be used.

The inventive subject matter being thus described, it will be obviousthat the same may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the inventivesubject matter, and all such modifications are intended to be includedwithin the scope of the following claims.

1. A thermally conductive adhesive composition comprising: a powder of ahigh melting point metal or metal alloy; a powder of a low melting pointmetal or metal alloy; and a polymerizable fluxing polymer matrixcomposition comprising a polyepoxide polymer resin and a low-meltingsolid or liquid acid-anhydride, wherein the polymerizable fluxingpolymer matrix composition further comprises: a polymer diluent with atleast one carbon carbon double bond; and a polymer diluent with at leastone functional hydroxyl group comprising at least one material selectedfrom the group consisting of: triethylene glycol, glycerol,tri(propylene glycol) methyl ether and di(ethylene glycol) butyl ether,wherein the ratio by weight of the powder of the low melting point metalor metal alloy to the powder of the high melting point metal or metalalloy ranges from about 0.50 to about 0.80.
 2. The thermally conductiveadhesive composition of claim 1, wherein the ratio by weight of thepowder of the low melting point metal or metal alloy to the powder ofthe high melting point metal or metal alloy is about 0.665.
 3. Thethermally conductive adhesive composition of claim 1, wherein thecomposition comprises 50%-60% by weight the powder of the high meltingpoint metal or metal alloy, and 30%-40% by weight the powder of the lowmelting point metal or metal alloy.
 4. The thermally conductive adhesivecomposition of claim 1, wherein the acid-anhydride comprises at leastone material selected from the group consisting of: tetrahydrophthalicanhydride, hexahydrophthalic anhydride, nadic methyl anhydride,4-methylhexahydrophthalic anhydride, methyltetrahydrophthalic anhydrideand mixtures thereof.
 5. The thermally conductive adhesive compositionof claim 1, wherein the polymer diluent with at least one carbon carbondouble bond comprises at least one material selected from the groupconsisting of: 1,6 hexanediol diacrylate; 1,6-hexanediol dimethacrylate;tris[2-(acryloxy)ethyl]isocyanurate; trimethylolpropane trimethacrylate;ethoxylated bisphenol diacrylate and mixtures thereof.
 6. The thermallyconductive adhesive composition of claim 1, wherein the polymerizablefluxing polymer matrix composition further comprises: a source ofradical initiators.
 7. The thermally conductive adhesive composition ofclaim 6, wherein the source of radical initiators comprises at least onematerial selected from the group consisting of: azobiscyclohexanecarbontrile, benzoyl peroxide, amyl peroxide,1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azbbisisobutyronitrile, andmixtures thereof.
 8. The thermally conductive adhesive composition ofclaim 1, wherein the powder of the high melting point metal or metalalloy comprises electronic grade copper.
 9. The thermally conductiveadhesive composition of claim 1, wherein the powder of the high meltingpoint metal or metal alloy comprises a material selected from the groupconsisting of copper, silver, aluminum, nickel, gold, platinum,palladium, beryllium, rhodium, nickel, cobalt, iron, molybdenum andalloys and mixtures thereof.
 10. The thermally conductive adhesivecomposition of claim 1, wherein the powder of the low melting pointmetal or metal alloy comprises a material selected from the groupconsisting of Sn, Bi, Pb, Cd, Zn, In, Te, Tl, Sb, Se, Ag, and alloys andmixtures thereof.
 11. The thermally conductive adhesive composition ofclaim 1, wherein the polymer diluent with at least one functionalhydroxyl group is present in sufficient amount to catalyzeesterification of the acid anhydride so as to produce a polymerizablefluxing agent during heating of the conductive adhesive.
 12. Thethermally conductive adhesive composition of claim 11, wherein thepolymer diluent with at least one functional hydroxyl group is presentin sufficient amount to aid in spreading or wetting of the polymerizablefluxing polymer matrix during heating of the conductive adhesive.